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Heat Flow: 7 Minutes of BS (#buildingscience)

Three mechanisms usually work together to move electrons from more to less. These three mechanisms can conspire to move heat AROUND your insulation
February 12, 2021

What heat flow is:

heat flow

hēt flō (n).

“Heat flow is the movement of heat energy between objects from a hot temperature to a cold temperature.”

And that is Jonathan Smegal, from RDH Building Science Laboratories.

We know that heat moves from hot to cold, but it's not always clear on how it moves.

So, it moves in one of three ways, either by conduction, convection, or radiation…

Conduction is direct heat transfer from one object to another by touching

More often than not, though, heat is moving in more than one way at once.

Conduction is probably the best understood of the three and is simply the transfer of heat through molecular contact, which is just a geeky way of saying two things of different temperatures that are touching and heat transfers from hot to cold.

For example, If you pick up a hot pan, the heat transfers to your fingers from the pan and you get burned.

If you stick your tongue against a cold metal pole in the winter, then heat from the surface of your tongue transfers very quickly, and your tongue freezes to the pole.

If you learn nothing else from this podcast, learn this: always, no wait, NEVER touch your tongue to a cold flagpole.

So that’s conduction, and as I said, it’s usually the easiest to understand.

Convection is a fluid process

Convection is a process of moving heat through fluids. In construction, those fluids are usually air or water.

In a lot of cases, we have pumps that move heated fluid into radiators or radiant floors.

Another example is using fan-coil units, or an air handler unit to push warm or cool air around the building to transfer heat that way.

So that’s the majority of the water part, let’s look at the air: the majority of convective activity through the building enclosure.

Even for what might seem like small airflows, especially in a well-insulated enclosure, this convection, this air movement across the building will dominate the heat loss across the enclosure. 

Dominate the heat loss across the enclosure. That’s an important point that answers the Why it matters question, but we’ll come back to that.

The fluid convection talked about earlier is what’s called forced convection, because there were pumps, fans, or some kind of power behind the movement.

Another example of convection would be what we call natural convection. So because warm air is less dense and cooler air is denser, we often get stratification of air and movement against surfaces.

Natural convection is when air moves based on the temperature difference. As the air cools, it will become denser, and it will fall and will be replaced with warmer less dense air.

For example, if you’re standing against a large window, the air in the room will cool as it comes in contact with that window and it will fall down the surface of the window, often leading to condensation in the corners…

This cold air movement can also make a room feel drafty. There is no actual air leaking through the window, but the cold air rushing past it makes the room feel drafty and the people feel cold.

Also within wall assemblies, within enclosures with air-permeable insulation, if you don’t do a good job installing insulation in your stud cavities, you can get convective looping in your stud cavities.

Just like the convective current that rolls past your window and drops condensation at the bottom, little convective loops in the stud cavities can mean little lumps of water in your walls. Puddles might be a bit over the top.

OK, that’s the high and low on convective heat flow: fluids carrying heat.

Radiant heat flows through space or clear objects

The last form of heat transfer is radiation.

Radiation heat transfer is a bit more complicated to understand because you can’t observe it in ways that you could observe conduction or convection.

Radiation can go through clear surfaces, such as windows, it can go through vacuums like space, and vacuum insulated panels, but it can not go through solid objects.

The most obvious, important, and probably easy to understand example of radiant heat flow is the sun. In the thermal-envelope-world, the heat can be either wanted or unwanted, depending on the season and where you live.

When it comes to radiation heat transfer and the opaque enclosure, in general people don’t understand radiative heat transfer as well, there can be confusion in how radiant barriers, foils, low e coatings on surfaces can work.

I happen to know that there is a whole podcast on the topic of radiant barriers, so dig into that if you’re still hungry.

Radiation heat transfer is just two surfaces of different temperatures that are radiating through space to try to even out the temperature difference between those two surfaces.

If you have a warm surface and a cold surface, and they can see each other, they will try to come to an equilibrium temperature by radiating heat or cold to each other

For example, when you stand next to a cold wall or window, you can feel the cold, even without touching it because your heat is radiated away. The cold wall literally sucks heat away from you.

So, if you touch the cold wall, that’s conduction. If you feel a cool draft coming off the wall, its convection. When you feel the heat sucked out of you, its radiation.

All three modes of heat flow work simultaneously in differing degrees

Even though all three of these modes are relatively straightforward, when it comes to the enclosure of buildings and enclosure components,

All three modes of heat flow work simultaneously. Usually, though one form is dominant over the others...

...which can make it difficult to understand exactly what’s going on with the heat transfer.

The three modes of heat transfer could change through a wall assembly, and end up acting in series, or even in parallel depending on the type and complexity of the wall assembly.

To illustrate, let’s look at a bare concrete wall with no insulation, above ground on a sunny day.

radiation from the sun coming down on the building and it heats up the surface of the concrete.

So that’s radiative heat transfer from the sun to the exterior surface of the concrete.

From there, the concrete gets quite warm, as you would expect, and the heat transfer through the concrete itself is almost entirely by conduction. There’s are very little space or air pockets inside it for conduction or radiative transfer.

But when the heat gets to the other side, it changes again

To radiation again, BUT,

It’s a different radiation, it’s in the infrared spectrum, a much lower temperature,

Whereas the radiation from the sun was in the Ultraviolet spectrum, a much hotter spectrum to sit in.

But the heat doesn’t stop with infrared radiation.

The heated interior surface of the concrete creates convection currents.

So that surface will heat up, it will cause the air adjacent to that surface to rise and you’ll get radiative and convective heat transfer off of that surface.

So just in the simple example of a concrete wall, we have examples of all three methods of heat transfer, working to move heat from the exterior to the interior space.

Heat can flow around wall insulation

In pretty much any place you’re going to build a concrete wall, there’s going to be a need to insulate that concrete wall.

A common easy to insulate that concrete wall is to put on steel studs, fiberglass batts, and interior gypsum board.

So, what happens then?

The sun heats the wall, the wall gets warm, and it radiates heat to the interior surface. Instead of radiating directly to the interior now, steel studs in direct contact with that concrete are warmed by conduction.

Concrete heats up, studs are in contact, we have molecular contact, and we have heat transfer to the studs.

In between the steel studs we do have a fiberglass batt, which does a pretty good job of insulating the concrete—it really limits the conduction, the convection, and the radiation between the studs.

But this is a prime example of thermal bridging around insulation and the R-value loss as a result of thermal bridging and steel studs.

All of the heat will be conducted around the insulation to the interior surface of the drywall, and that is where you’ll have the connective and radiation heat transfer.

To reiterate, the majority all of the heat flows around the insulation through the steel studs and it heats up the drywall.

Heat is like a shape-shifting opportunist, taking advantage of whatever avenue it can to transport electrons.

The best way to build depends on where you're building

All three types of heat flow work together to move heat, and each type’s proportion of importance depends on

  • The assembly,
  • Where the heat is located within in the assembly,
  • and also where the assembly is located within the world.

In colder climates, heat flow reductions are typically done by specifying higher R-values of insulation and higher airtightness values to minimize both the largest sources of conduction and convection heat loses.

Windows are predominantly important for heat loss in cold climates, whereas in hot climates, they are the main source of heat gain, which overloads AC equipment.

In warmer more southern climates, solar control is typically the key to thermal control and comfort. Air tightness is important, but as a generalization, air tightness is not as critical, and the R-value levels do not need to be as high to maintain interior comfort.

Radiant barriers in the roof assembly and shading, low-e coatings on the windows are how heat flow is slowed.

Buildings with good thermal control are comfortable and inexpensive to operate for decades

So knowing how to stop the flow is one aspect of solving the puzzle. Knowing why helps to dial in the strategy.

The biggest reason to control heat flow in buildings is for occupancy comfort.

If you can control drafts, you can eliminate complaints about drafts.

You can also reduce condensation problems, which can lead to musty air and rotten wood,

Controlling heat flow also saves money for whoever pays the utility bills. A better thermal envelope means that you can reduce the size of the HVAC equipment that heats and cools the building.

The operational energy savings are critical because once you finish the building, they’re the main costs that you have to operate that building for the entire life of the building.

So if you can minimize the operational costs right off the batt, then for fifty, seventy-five, a hundred years, you’re gonna keep saving money year after year.

That’s a pretty smart approach to building design.

And it turns out, you get paid for what you do and what you know. Because when you know more, you can do more.

I want to thank Jonathan Smegal and RDH Building Science Laboratories for providing Jonathan to participate in this podcast.

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